Method for polymerizing vinyl monomers

ABSTRACT

Vinyl monomers are polymerized in the aqueous phase in the presence of at least one N-oxyl compound of a secondary amine, having no hydrogen atoms bonded to the α carbon atoms. This process substantially eliminates deposits on reactor surfaces.

POLYMERIZATION OF VINYL MONOMERS

The present invention relates to the use of N-oxyl compounds for preventing wall deposits during the addition polymerization of vinyl monomers in aqueous phase.

In the course of polymerization reactions it is common for polymer deposits to appear on the reactor walls. These polymer deposits cause processing difficulties since they have to be removed at regular intervals, generally after each polymerization reaction, often entailing considerable expense. Moreover the deposits, which generally consist of polymers that are atypical in terms, for example, of their molecular weight distribution, can in some cases become detached and so lead to inhomogeneities in the polymers.

Various measures have been developed to remedy the formation of deposits. Since the extent of formation of deposits depends heavily on the surface structure of the reactor walls—their roughness, for example—the measures are aimed primarily at smoothing the surfaces. In addition to mechanical measures attempts have also been made to maintain the quality of the surface by adding corrosion inhibitors to the polymerization (see, for example, G. W. Becker, D. Braun (eds.), Kunststoff Handbuch, Hanser Verlag, Munich, Vienna 1986, Volume 2/1, p. 153). These measures, however, are normally unable to adequately prevent the formation of deposits.

The earlier German Patent Application 19 609 312.0 describes a process for preventing the premature polymerization of certain vinyl monomers in the course, for example, of their distillation, purification, storage and transportation, by adding N-oxyl compounds of secondary amines. The use of N-oxyl compounds has also been described for stabilizing styrene and other vinylaromatic compounds in the course of distillation (U.S. Pat. No. 5,254,760). It is also known, however, that even traces of such nitroxyl compounds disrupt the subsequent polymerization process; they cause retarded polymerization and uncontrolled chain termination, resulting in polymers deficient in reproducibility and of short chain length. These deleterious effects are described by Mardare et al. in Polym. Prep. (Am. Chem. Soc., Div. Polym. Sci.) 35 (1), 778 (1994).

It is an object of the present invention to find a vinyl monomer polymerization process in which the formation of wall deposits is substantially eliminated without perceptibly affecting the polymerization process.

We have found that this object is achieved by the use of N-oxyl compounds of secondary amines which carry no hydrogens on the a carbons to prevent deposits on reactor surfaces during the polymerization of vinyl monomers in aqueous phase.

By vinyl monomers are meant all addition-polymerizable monomers which carry a terminal olefinic double bond. Examples that may be mentioned are acrylic and methacrylic acid and their derivatives such as nitrites and esters, especially methyl, ethyl, propyl and butyl acrylate and methacrylate, and also vinyl esters of C₂-C₅ carboxylic acids, especially vinyl acetate, vinyl propionate, vinyl butanoate, aromatic vinyl compounds, especially styrene, dienes, especially butadiene, vinyl halides, especially vinyl chloride, vinyl ethers, such as methyl, ethyl or butyl vinyl ether, vinyl thioethers, vinylcarbazoles, vinylpyrrolidones, vinylphthalimides, vinyl isocyanates, vinylcaprolactams, vinylimidazoles, vinylformamide, vinylsulfonic acid, and vinyl silanes such as vinyltriacetoxysilane, vinyltrichlorosilane or vinyltrimethoxysilane.

α-Olefins are also suitable, such as ethylene, propylene, 1-butene, 1-hexene and 1-octene, especially together with other comonomers in copolymerization processes.

The process of the invention can be employed to particular advantage in the case of such copolymerization processes. Examples that may be mentioned of suitable copolymers which can be prepared by the processes are styrene-butadiene, synthetic rubberlike block polymers, such as styrene-butadiene-styrene, styrene-butyl acrylate, butadiene-acrylonitrile, acrylonitrile-butadiene-styrene, and ethylene-vinyl chloride.

The use according to the invention is particularly suitable in the polymerization of styrene, alone or together with further comonomers, and, if desired, in the presence of volatile blowing agents, such as pentane, and for the polymerization of vinyl chloride.

The process of the invention relates to polymerizations in aqueous phase. The term aqueous phase is intended to denote solutions, emulsions and suspensions of the corresponding monomers or polymers in water and in solvent mixtures comprising a substantial proportion , i.e., at least 20% by weight, of water. All customary emulsion polymerizations and suspension polymerizations, therefore, can be conducted in accordance with the process of the invention. The process conditions which can be chosen are those which are customary for such polymerization processes and are described, for example, in Houben Weyl, Methoden der organischen Chemie, Vol. E20, p. 218 ff and in Ullmanns Encyklopäldie der technischen Chemie, 4th edition, Vol. 19, p. 132 ff. and the literature cited therein.

In accordance with the invention, N-oxyl compounds of secondary amines are present during the polymerization process. The application of these N-oxyl compounds can be made in various ways.

It has been found advantageous to add the N-oxyl compound to the polymerization mixture. Alternatively, the N-oxyl compound may already be present in the monomer in order to protect it against premature polymerization.

The concentration of the N-oxyl compound in the polymerization mixture should be such that it has little effect on the rate of polymerization. The sensitivity of the polymerization to the amount of N-oxyl compound depends on a variety of parameters: in particular, on the nature of the monomer, the structure of the N-oxyl compound, the temperature, and other free-radical initiators and free-radical scavengers which may be present in the reaction mixture. The optimum concentration, however, can be determined in a few preliminary experiments under given process parameters.

A concentration range which has been found advantageous for the N-oxyl compound(s) in the polymerization mixture is that from 0.5 to 50 ppm, in particular from 1 to 20 ppm and, especially, from 2 to 10 ppm (1 ppm=10⁻⁶ parts by weight of N-oxyl compound based on the overall weight of the monomers). At these concentrations, the formation of deposits is generally inhibited completely without any notable effect on the kinetics of the reaction.

In addition to adding the N-oxyl compound to the polymerization mixture, a process variant which has proven particularly appropriate is that in which the reactor surface is wetted with a solution of the N-oxyl compound before the reactor is filled with the polymerization mixture.

Wetting can be carried out simply and effectively by spraying the reactor walls and other components within the reactor, such as the stirrer, with a solution of the N-oxyl compound. The N-oxyl compounds are in many cases of poor solubility in water but dissolve for the most part in organic solvents such as methanol, ethanol, propanol, acetone, ethyl acetate, dimethylformamide, etc. A particularly appropriate solvent for many N-oxyl compounds is methanol. The concentration of the N-oxyl compound in the spray solution is not critical and is advantageously set at between 0.01 and 1% by weight, based on the overall mass of the spray solution.

Alternatively, the reactor surface can be wetted by filling it with a solution similar to the spray solution and then draining off that solution.

Wetting of the reactor surface with the N-oxyl compound has the advantage over its addition to the polymerization mixture that the effect on the rate of polymerization is extremely small and, in addition, that the region above the liquid level, especially the region just above the liquid level, in which deposits are common, is also protected against the formation of deposits.

It is also advantageous to combine both forms of application, in which case very small amounts are sufficient for direct addition to the polymerization solution.

Suitable N-oxyl compounds for use in the process of the invention are, for example, those having the following structures

where R is identically or differently alkyl, cycloalkyl, aralkyl or aryl radicals, which may also be linked in pairs to form a ring system, and Y is a group required to complete a 5- or 6-membered ring. For example, R is a C₁-C₂₀-, in particular a C₁-C₈-alkyl radical, a C₅- or C₆-cycloalkyl radical, a benzyl radical or a phenyl radical. Y is, for example, an alkylene group —(CH₂)₂— or —(CH₂)₃—.

Also suitable are N-oxyl compounds such as the following structures

where the aromatic rings may each carry from 1 to 3 inert substituents, such as C₁-C₄-alkyl, C₁-C₄-alkoxy or cyano.

It is preferred to employ sterically hindered amine derivatives of cyclic amines, such as of piperidine or pyrrolidine compounds, which may include a further heteroatom in the ring, such as nitrogen, oxygen or sulfur, said heteroatom not being vicinal to the hindered amine nitrogen. The steric hindrance is provided by substituents in both vicinal positions to the amine nitrogen, suitable substituents being hydrocarbon radicals, which replace all 4 hydrogens of the α—CH₂ groups. Examples of substituents are phenyl, C₃-C₆-cycloalkyl, benzyl and in particular C₁-C₆-alkyl radicals, in which case the alkyl radicals attached to the same a carbon may be linked with one another to form a 5- or 6-membered ring. Particularly preferred radicals are those given individually for R¹ and R². Preferred N-oxyls of sterically hindered amines that are employed are derivatives of 2,2,6,6-tetraalkylpiperidine.

Preferred N-oxyl compounds for use in the process of the invention are those of the general formula I

where

R¹ and R² are C₁-C₄-alkyl or phenyl or, together with the carbon to which they are attached, are a 5- or 6-membered saturated hydrocarbon ring,

R³ is hydrogen, hydroxyl, amino, SO₃H, SO₃M, PO₃H₂, PO₃HM, PO₃M₂, organosilicon radicals or an m-valent organic radical which is attached via oxygen or nitrogen, or, together with R⁴, is oxygen or a ring structure defined under R⁴, where M is an alkali metal,

R⁴ is hydrogen or C₁-C₁₂-alkyl or, together with R³, is oxygen or, together with R³ and the carbon to which they are attached, is a ring structure

 where, if R³ and R⁴ join to form a radcial, m is 1,

R⁵ is hydrogen, C₁-C₁₂-alkyl or —(CH₂)_(z)—COOR⁶,

R⁶ is identical or different C₁-C₁₈-alkyl,

k is 0 or 1

z, p are from 1 to 12, and

m is from 1 to 100.

R¹ and R² can, for example, be methyl, ethyl, propyl or butyl or together can form a tetra- or pentamethylene group. Preferably, R¹ and R² are methyl groups.

R⁴ is suitably, for example, hydrogen, the abovementioned C_(l)-C₄-alkyl groups, and also pentyl, sec-pentyl, tert-pentyl, neopentyl, hexyl, 2-methylpentyl, heptyl, 2-methylhexyl, octyl, isooctyl, 2-ethylhexyl, nonyl, 2-methylnonyl, isononyl, 2-methyloctyl, decyl, isodecyl, 2-methylnonyl, undecyl, isoundecyl, dodecyl and isododecyl, (the names isooctyl, isononyl and isodecyl are trivial names and derive from the carbonyl compounds obtained by oxo synthesis; cf. in this regard Ullmann∝s Encyclopedia of Industrial Chemistry, 5th Edition, Vol. Al. pages 290-293, and also Vol. A10, pages 284 and 285).

p is preferably 6-12, more preferably 9.

z is preferably 1-4, more preferably 2.

Other than hydrogen, R⁵ is suitably, for example, the abovementioned C₁-C₁₂-alkyl groups. Preferably, R⁵ is hydrogen, C₁-C₄-alkyl or (CH₂)_(z)—COO(C₁-C₆-alkyl), more preferably the radicals —CH₂—CH₂—COO(CH₂)₁₁—H₃ and —CH₂—CH₂—COO(CH₂)₁₃—CH₃.

R⁶ can, for example, be one of the abovementioned C₁-l₂-alkyl groups or tridecyl, isotridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl or octadecyl. Dodecyl and hexadecyl are preferred.

Preferred radicals R³ are, for example, the following m-valent radicals

where

R⁷ is C₁-C₁₂-alkyl or —(CH₂)_(z)—COOR⁶

R⁸ is hydrogen or C₁-C₁₈-alkyl,

R⁹ is C₁-C₁₈-alkyl, vinyl or isopropenyl,

R¹⁰ is C₈-C₂₂-alkyl,

R₁₁ is hydrogen or an organic radical as normally formed in the free-radical polymerization of the initial monomers,

k is 0 or 1,

x is from 1 to 12 and

n is an even number m.

If R³ is one of these radicals then R⁴ is preferably hydrogen. The variable m in this case can be from 1 to 100. m is preferably 1, 2, 3, 4 or a number from 10 to 50, mixtures generally being employed especially in the case of the oligomeric or polymeric radicals R³.

Suitable radicals R⁷ are the same as those specified for R⁵. R⁷ is preferably C₁-C₄-alkyl.

Other than hydrogen, suitable radicals R⁸ are the same as those specified for R⁶. R⁸ is preferably hydrogen.

R⁹ is suitably, in particular, vinyl, isopropenyl or C₁₅-C₁₇-alkyl radicals.

Examples of suitable radicals R¹⁰ are the above-mentioned C₈-C₁₈-alkyl radicals and also nonadecyl, eicosyl, uneicosyl and doeicosyl. In this case preference is given to mixtures of different radicals R¹⁰ differing in the length of the carbon chain.

The radicals R¹¹ are hydrogen or organic radicals as formed in the free-radical polymerization of the initial monomers: in this case, of an ethylene derivative and a maleimide derivative; in other words, for example, a radical formed from the polymerization initiator or from a free radical occurring as an intermediate, or another such radical, as is familiar to the person skilled in the art.

It is also possible with advantage to employ nitroxyl compounds of the formula (Ia)

where

R¹ and R² are as defined above,

R¹² is an m′-valent radical attached via carbon, oxygen or nitrogen,

R¹³ is hydrogen, C₁-C₁₂-alkyl or C₁-C₁₂-alkoxy or, together with R¹², is an m′-valent radical attached via carbon or oxygen by a chemical double bond to the carbon which carries these groups, or, together with R¹² and the carbon which carries these groups, is a saturated isocyclic or heterocyclic, 3- to 7-membered ring, in which case m is 1, and

m′ is 1, 2 or 3.

Suitable m-valent radicals R¹² are C₁-C₄-alkyl, unsubstituted phenyl and phenyl substituted by from one to three C₁-C₄-alkyl radicals, examples of C₁-C₄-alkyl radicals having already been set out above. The attachment of these radicals to the piperidine ring can be via oxygen, an NH group or an N(C₁-C₄-alkyl) group.

Examples of possible radicals R¹² (where the lines denote the free valences) are:

The C₁-C₁₂-alkyl and C₁-C₁₂-alkoxy groups which are possible representatives of the radicals R¹³ have already been addressed in an exemplary manner above for the radicals R⁴. The radicals R¹² and R¹³ may also together form a group which then is attached through a chemical double bond via carbon or nitrogen to the carbon which carries the groups (the carbon in position 4 of the piperidine ring). Examples of such 2 m′-valent groups (where the lines denote the free valences) can be:

It is also possible for the radicals R¹² and R¹³, with the carbons carrying these groups, to form a 3- to 7-membered isocyclic or heterocyclic ring.

Examples of such rings are, for instance:

The groups R¹⁴ in these groups can be identical or different and are hydrogen, C₁-C₁₂-alkyl, unsubstituted phenyl, or phenyl substituted by from one to four C₁-C₄-alkyl groups. Examples of corresponding C₁-C₁₂-alkyl groups, and C₁-C₄-alkyl groups which may occur as substituents on the phenyl ring, have already been indicated above. The variable k′ can adopt a value of 0,1 or 2. C* denotes the carbon in position 4 of the piperidine, which is incorporated into the ring system.

Other preferred nitroxyl compounds are the following:

1-oxyl-2,2,6,6-tetramethylpiperidine,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-ol,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-one,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 2-ethylhexanoate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 4-tert-butylbenzoate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) succinate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) adipate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n-butylmalonate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) phthalate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) isophthalate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) terephthalate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) hexahydroterephthalate,

N,N′-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide,

N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)caprolactam,

N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)dodecylsuccinimide,

2,4,6-tris[N-butyl-N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl]-s-triazine,

4,4′-ethylenebis(1-oxyl-2,2,6,6-tetramethylpiperazin-3-one), and tris(2,2,6,6-tetramethyl-1-oxylpiperidin-4-yl) phosphite.

In the process of the invention it has been found particularly advantageous to use N,N′-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N′-bis-formyl-1,6-diaminohexane as the N-oxyl compound.

The nitroxyl compounds described can be prepared from the corresponding piperidine compounds by oxidation with hydrogen peroxide, for example. Details of this oxidation are given, for example, in the earlier German Patent Application 195 101 84.7. The secondary amines which carry no hydrogens on the α carbons, such as piperidine compounds, and their preparation are common knowledge.

EXAMPLES 1 TO 3

A 3 1 steel autoclave was filled with

1300 g of water

10.5 g of 4% by weight polyvinyl alcohol solution, degree of saponification 88%

0.34 g of 40% strength by weight polyvinyl alcohol solution, degree of saponification 47%

11.65 g of 3% strength by weight hydroxymethylpropylcellulose solution

0.56 g of tert-butyl perneodecanoate

1.75 g of sodium tripolyphosphate

and with varying amounts (see table) of N,N′-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N′-bis-formyl-1,6-diaminohexane. The autoclave was closed, flushed with nitrogen, examined for leaks, and evacuated. Then 700 g of vinyl chloride were introduced. The autoclave was heated to 55° C. Polymerization was continued until the pressure had fallen by 4 bar, and then the autoclave was let down. The results are given in the table.

EXAMPLE 4

A 0.1% strength solution of N,N′-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N′-bis-formyl-1,6-diaminohexane in methanol was sprayed uniformly onto the inner surface of the autoclave and dried at room temperature. The autoclave was charged, and the reaction conducted, exactly as described in Examples 1 to 3 but without the addition of N,N′-bis-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N′-bis-formyl-1,6-diaminohexane. The result is indicated in the table.

Noninventive, comparative example

An autoclave was charged, and the reaction conducted, exactly as described in Example 4 but without the surface being sprayed. The result is indicated in the table.

TABLE Run time Example N-oxyl min. Deposit 1 10 ppm 269 Deposit only above the liquid level 2 20 ppm 332 Deposit only above the liquid level 3 50 ppm 405 Deposit only above the liquid level 4 * 264 No deposit Comparative — 256 Deposit everywhere; removable only by mechanical means * Autoclave sprayed with N,N′-bis-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N′-bisformyl-1,6-diaminohexane solution

* Autoclave sprayed with N,N′-bis-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N′-bisformyl-1,6-diaminohexane solution 

We claim:
 1. A process for polymerizing a polymerization mixture of vinyl chloride in aqueous phase in a reactor, which comprises polymerizing the vinyl chloride in the presence of at least one N-oxyl compound of a secondary amine which carries no hydrogens on the α carbons.
 2. A process as claimed in claim 1, wherein the N-oxyl compound is added to the polymerization mixture.
 3. A process as claimed in claim 1, wherein the N-oxyl compound is added to the polymerization mixture in a concentration of from 0.5 to 50 ppm.
 4. A process as claimed in claim 1, wherein the surfaces of the reactor are wetted with a solution of the N-oxyl compound before the reactor is filled with the polymerization mixture.
 5. A process as claimed in claim 1, wherein the N-oxyl compound is represented by formula I:

wherein R¹ and R² are C₁-C₄-alkyl or phenyl or, together with the carbon to which they are attached, are a 5- or 6-membered saturated hydrocarbon ring, R³ is hydrogen, hydroxyl, amino, SO₃H, SO₃M, PO₃H₂, PO₃HM, PO₃M₂, organosilicon radicals or an m-valent organic radical which is attached via oxygen or nitrogen, or, together with R⁴, is oxygen or a ring structure defined under R⁴, wherein M is an alkali metal, R⁴ is hydrogen or C₁-C₁₂-alkyl or, together with R³, is oxygen or, together with R³ and the carbon to which they are attached, is a ring structure

wherein, if R³ and R⁴join to form a radical, m is 1, R⁵ is hydrogen, C₁-C₁₂-alkyl or —(CH₂)_(z)—COOR⁶, R⁶ is identical or different C₁-C₁₈-alkyl, k is 0 or 1 z, p are from 1 to 12, and m is from 1 to
 100. 6. A process as claimed in claim 5, wherein the radical R³ in formula I is a radical represented by the formula:

wherein R⁷ is C₁-C₁₂-alkyl or —(CH₂)_(z)—COOR⁶ R⁸ is hydrogen or C₁-C₁₈-alkyl, R⁹ is C₁-C₁₈-alkyl, vinyl or isopropenyl, R¹⁰ is C₈-C₂₂-alkyl, R¹¹ is hydrogen or an organic radical as normally formed in a free-radical polymerization of the vinyl chloride, k is 0or 1, x is from 1 to 12 and n is an even number from 1 to
 100. 7. A process as claimed in claim 1, wherein the N-oxyl compound is N,N′-bis(1oxyl-2,2,6,6,-tetramethyl-piperidin-4-yl)-N,N′-bis-formyl-1,6-diaminohexane. 